An air bag device is one for protecting an occupant upon deploying an air bag when a vehicle comes into collision.
A conventional air bag is usually made of woven fabric of fiber, such as polyamide fiber, the surface of which is coated with silicone rubber. Instead, an air bag made of resin films (Japanese Unexamined Patent Publication No. 2 (1990)-3 1965) and an air bag made of elastomer (Japanese Unexamined Patent Publication No. 4 (1992)-266544) have been proposed recently.
Particularly, an air bag made of thermoplastic resin films can be easily and efficiently manufactured upon fusing a plurality of thermoplastic resin films together by heat sealing, thereby allowing the simple manufacturing process. Therefore, it is quite effective from the industrial point of view.
The air bag made of thermoplastic resin films however has a problem that the thermoplastic resin films are easy to break along an edge of the fused portion when the air bag is deployed.
That is, as shown in FIG. 10, when two thermoplastic resin films 31, 32 are superposed and then fused together by heat sealing, the thermoplastic resin films are compressed at a fused portion 33 so that the thickness of the fused portion 33 is decreased. In addition, in cases of some films, the hardness of fused portion 33 is increased due to the fusing on the surface. There are portions 34, having a reduced thickness and strongly affected by heat, between the fused portion 33 and portions 35 not fused. While the thickness of the portions 35 is increased upon compressing the films 31 and 32 so that the strength of the portions 35 is increased, the strength of the portions 34 is relatively decreased due to its reduced thickness. When the air bag consisting of the thermoplastic resin films 31 and 32 fused together by heat sealing as mentioned above is deployed by great gas pressure from an inflator, as shown in FIG. 10, the portions 34 have a reduced thickness and the less strength which are boundaries between the fused portion 33 and the portion 35 having an increased thickness or its original thickness and thereby the original strength. Therefore, the portions 34 are stressed the most. In addition, since the thermoplastic resin films 31 and 32 are deployed while bending, the thermoplastic resin films 31 and 32 are easy to break along the boundaries (shown by a broken line in FIG. 10).